Managing payload specific latencies in a cross-connect system

ABSTRACT

A method and apparatus for managing latency includes receiving data from synchronous transport signals (STS) and virtual tributary (VT) sources, providing a provisioning bit for each output associated with a memory, and adjusting a pointer for the VT sources based on the provisioning bit such that the STS and VT outputs are synchronized.

BACKGROUND

This invention relates to managing payload specific latencies in across-connect system.

In a synchronous transmission system or network, data traffic takes theform of synchronous data signals comprised of data frames. Data trafficis transported in high order or low order data structures, where a highorder structure may include more than one low order structure. Forexample, a synchronous digital hierarchy (SDH) system carries data instructures known as virtual containers (VCs), which may be high ordercontainers, for example a VC-4, or low order containers, for example aVC-11 or VC-12. For example, a VC-4 may include 63 VC-12s or two loworder VC-3s and 21 VC-12s. The VCs are transported across a network inframes known as synchronous transport module (STM) frames. Similarly, asynchronous optical network (SONET) system carries data in high orderstructures known as synchronous transport signals (STS) or low orderstructures known as virtual tributaries (VTs). An STS-1 may include somecombination of lower signals such as 28 VT1.5s or 21 VT2s, where VT2sare comparable with SDH VC-12s. VC-11s and VC-12s are transported intributary units TU-11 and TU-12 respectively.

In some cases, is not desirable for network equipment to provide fulllow order switching. It is, however, desirable for network equipment toprovide some low order switching of VT (VT1.5/VT2) and/or TU (TU11/TU12)rate signals. This is commonly known as partial VT/TU switching. Asystem including both a VT/TU switching apparatus and an STS switchingapparatus provides partial VT/TU switching. In such an arrangement, thehigh-order (STS/VC-3/VC-4) switch passes to the VT/TU switch only VT/TUtraffic that requires switching (where switching normally includestimeslot interchanging and/or port interchanging).

SUMMARY

In one aspect, the invention features a method and apparatus formanaging latency. The method includes receiving data from high-ordersynchronous transport module (STM) and synchronous transport signal(STS) sources and low-order tributary unit (TU) and virtual tributary(VT) sources, providing a provisioning bit for each output associatedwith a memory, and adjusting a pointer for the low-order (VT/TU) sourcesbased on the provisioning bit such that the high-order(STS/STM/AU-5/AU-3) and low-order (VT/TU) outputs are synchronized.

One of more of the following features may also be included. The methodmay include determining an adjustment for the pointer based on apredetermined delay or adjusting the pointer by a predetermined numberof time slots. The method may include assembling synchronized outputsfrom the memory. The method may include adjusting the high-order pointerto synchronize the output of the low-order and high-order sources, wherethe high-order sources incur less delay to pass through the memory thanlow-order sources. Providing a provisioning bit can include storing theprovisioning bit in a connection memory. The method includes adjusting apointer for the low-order sources based on the provisioning bit. Themethod includes setting the provisioning bit for low order sources andadjusting the pointer when the provisioning bit is set. The methodincludes setting the provisioning bit for high-order sources and notadjusting the pointer when the provisioning bit is-not set.

One or more aspects of the invention may provide one or more of thefollowing advantages.

The use of a provisioning bit allows a user to set the expected signaltype and switch the signal according to the type. For high-order signalssuch as an STS signal that are to be groomed or switched at thelow-order level such as a VT signal, the pointer is adjusted allowingsynchronization of the outputs from the cross-connect device.

Typically, signals switched at the high-order by a high-order switchincur less delay than low-order signals that are switched by a low-orderswitch. A low-order switch is capable of grooming higher-order signalsas a concatenation of low-order signals and overhead, however, thehigh-order signals incur the same delay as the low-order signals.

A technique for switching both types of signals is to subtend alow-order switch from a high-order switch. Using this technique, thehigh-order switch forwards only the signals that require furthergrooming to the low-order switch. The signals groomed at the low-orderincur more delay than those groomed only at the high-order. Using thistechnique at least two switch devices or stages are required and twosets of connections need to be provisioned; one for the high-ordercross-connect and one for the low-order cross-connect. If a singledevice includes both functions, latency can be managed by signal type,provisioning complexity can be reduced and memory resources could bebetter managed.

Since the amount of delay incurred in a signal is proportional to theamount of data memory required to buffer the signals for switching, aswitch with a limited amount of memory can dynamically (or staticallyunder user control) partition the memory or memories to accommodateswitching some portion of the data as STS-only signals and toaccommodate some other portion for VT switching. Typically, the inputbandwidth of a switch capable of VT switching is limited to the amountof memory under its control. By efficiently using larger segments ofmemory for VT switching and smaller segments for STS switching, themaximum input bandwidth becomes a function of the proportion of thebandwidth (and thus memory) that is being used for VT switching.

The ability to swap a first connection RAM and a second connection RAMprovides system flexibility by allowing a user to switch the expectedsignal type (indicated by the provisioning bit) while the cross-connectis in operation.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a network.

FIG. 2 is a block diagram depicting a cross-connect system.

FIG. 3 is a block diagram of a switching core in an STS/VTcross-connect.

FIG. 4 is a block diagram of mapping for VT and STS signals.

DETAILED DESCRIPTION

Referring to FIG. 1, a network 10 includes a plurality of networkelements 12 in communication using a network infrastructure 14. Thenetwork infrastructure 14 may include network elements or nodes, and/orsub-networks. The network elements include synchronous transmissionapparatus such as a multiplexer, a regenerator, or a cross-connect (notshown). In one example, the network can be a synchronous optical network(SONET) and/or synchronous digital hierarchy (SDH) network. Networkelements 12 in network 10 switch data traffic.

An application specific integrated circuit (ASIC) providescross-connection and protocol conversion functions. The ASIC includes arandom access memory (RAM) that provides a cross-connection between aplurality of input ports and a corresponding plurality of output ports.An associated core logic controls the writing of incoming data to theRAM and subsequent reading of outgoing data from the RAM to providecross-connection, format conversion, and channel reorderingfunctionality.

In SDH networks, it is desirable for a switching element to control datapayloads at the high order level (e.g., at STS, VC-3 or VC-4 level) andat the low order level (e.g., at VT1.5, VT2, TU-11 or TU-12 level). In aSONET network, high order switching is referred to as STS switching, anda high order switch is referred to as an STS switch, while low orderswitching is referred to as VT switching and a low order switch isreferred to as a VT switch. It will be understood however that theinvention applies to equivalent SDH, and other, structures and apparatus(e.g. TU switching and TU alignment).

Referring to FIG. 2, a cross-connect system 200 includes a switch 201.In this example, switch 201 includes two switching stages: a first stage204 switches high-order signals (e.g. an STS switch) and a second stage202 switches low-order signals (e.g., a VT switch). The low-order switch202 is logically subtended from a high-order switch 204 (e.g., the STSswitch). When a low-order switch (such as a VT switch) is “subtended”from a high-order switch (such as an STS switch) the high-order switchpasses to the low-order switch only portions of an incoming data streamcomprising low-order data to be switched (where switching normally takesthe form of timeslot interchanging and/or port interchanging) all otherdata in the incoming data stream is handled by the high-order switchalone.

The cross-connect device 200 also includes a link terminator 206 forrespective pairs of input 208 and output 210 communication lines (e.g.,optical fibers). The link terminator 206 receives incoming data streamsfrom the input communication line 208, and sends switched outgoing datastreams onto the output communication line 210 or to an end user orother network element (not shown). The link terminator 206 is connectedto a low-order aligner 212 that aligns an incoming data stream to alocal timing reference and aligns an incoming data frame to a referencedata frame structure determined by the local timing reference. Thelow-order aligner also performs low-order pointer processing operationsas defined in applicable standards such as the InternationalTelecommunications Union Telecommunications Standardization Sector(ITU-T) standard G707 and Telcordia standard GR253.

Referring to FIG. 2, it is possible to combine the high-order switchingfunction 204 and low-order switching function 202 into a single device.This allows for high-order type signals to be switched with low latencyand the low-order type signals to be switched with a higher latency.

Referring to FIG. 3, a slice of the switching core 20 includes an STSinput mode control circuit 29 and write address generator 26 thatcontrols a flow for writing the data on input bus 21 into the datamemory block 23. Memory block 23 includes one or more RAMs (not shown).The STS input mode control circuit 29 is capable of storing a binary bitof information for every STS input of the switch. Switching core 20 alsoincludes a connection random access memory (RAM) 28 capable of storing abinary bit (provisioning bit 30) of information for every STS output ofa VT switch device. Provisioning bit 30 specifies which outputs aresourced as STS connections. The switching core also includes a datamemory block 23 that includes one or more RAMs for switching data.Connection RAM 28 controls reading of data out of memory block 23. Theslice of the switching core 20 also includes other blocks to manage theswitching of data. For example, the switching core 20 includes a swapcontrol module 22, a read access generator 24, and a read addresstranslator 27. The read address translator 27 can be used to map ortranslate the read addresses emerging from the read address generator24. Translation may be required if the memory within the data memoryblock 23 is allocated based on signal type as per the provisioningstored in the STS input mode control block 29.

In VT and STS switches, the switch receives incoming data, writes thedata into an internal memory, stores the data for a period of time, andsubsequently reads the data from the memory. Factors such as transitdelays through the switching apparatus, for example delays associatedwith pointer processing, contribute to the delay between an input andoutput of the switch. Consequently, VT and STS switches introduce adelay between a data frame structure received at an input of the switchand a data frame structure delivered on an output of the switch. The VTswitch provides data to the STS switch on the same timing as the otherinputs to the STS switch, but a data frame from a VT source incurs agreater delay than a data frame from an STS source. Hence, withoutpointer adjustment or VT alignment, a data frame provided to the STSswitch from the output of the VT switch would arrive a number of clockcycles after it was required. Typically, for the STS switch to workcorrectly, the data frame is supplied before it is needed on the output.

In one example, to ensure STS and VT frames arrive together, the framessupplied to the switch that are to be switched at the VT level havetheir payload advanced by a predetermined number of clock cycles tocompensate for the delays associated with VT switching. The whole inputframe is not simply advanced, because certain components of the frame,including the high order pointers and section/line overhead shouldoccupy a fixed position in time with respect to the timing reference ofthe local node. Such components of the frame are known as fixedoverhead. However, the VT traffic (or other low order traffic) isadvanced with respect to the frame so that the VT traffic arrives at theswitch advanced by an appropriate number of clock cycles to compensatefor switch delays. This is achieved by adjusting the high order pointervalues (e.g. the AU-4 pointers in SDH or the STS-1 pointers in SONET) torelocate the position of the low order pointers (e.g. the TU pointers orVT pointers).

In a system where both STS and VT sources are received, the VT sourceswill incur a greater delay when passing through the switching core 20due to a de-multiplexing stage for VT signals switched at the VT level.The STS-only connections incur a minimum latency when cross-connectedthrough the device. In order for the switch to assemble synchronizedoutputs that can be a mix of STS-only and VT sources, the pointer forinput frames which include VT sources is adjusted as described above.Provisioning bit 30 distinguishes VT sources from STS-only sources. A“set” provisioning bit 30 indicates a VT source and the system willadjust the pointer accordingly. If the provisioning bit is “not set”,the source is an STS-only source and the pointer is not adjusted.

In another example, frames that will be switched at the VT level do nothave their payload advanced. Instead, they incur delay through theswitching core due to VT switching.

Provisioning bit 30 is set to distinguish VT sources from STS-onlysources. Before being applied to the switch output, where frames shouldbe synchronized to frames switched as lower latency STS signals, theVT-switched frames have their overhead corrected and pointer offsetadjusted such that the fixed overhead aligns between all output frames.As in the previous example, this is achieved by adjusting the high orderpointer values (e.g. the AU-4 pointers in SDH or the STS-1 pointers inSONET) to relocate the position of the low order pointers (e.g. the TUpointers or VT pointers).

In both these examples, the provisioning bit 30 is set to indicate a VTsource and the provisioning bit is not set to indicate an STS-onlysource the state of the bits could be changed. If the state of the bitsis changed, the provisioning bit 30 is set to indicate an STS-onlysource and the provisioning bit is not set to indicate a VT source.

Provisioning bit 30 can be set before the cross-connect is in use, orcan be set while the cross-connect is in operation. To set theprovisioning bit 30 while in operation, a user sets the control bit 30in a duplicate connection RAM 28 and swaps control of reading andwriting to the duplicate connection RAM 28 using swap control module 22.The ability to swap connection RAM 28 and connection RAM 28 providessystem flexibility by allowing a user to switch the expected signal type(indicated by the provisioning bit 30) while the cross-connect is inoperation.

FIG. 4 shows an example mapping 50 including a VT input 52 to theswitch, a VT output 62 of the switch, an STS input 64 to the switch, andan STS output 70 from the switch. As described in one example above, inorder for the switch to assemble synchronized outputs that includeSTS-only and VT sources with associated latencies, the input framesincluding VT sources have their pointer adjusted at the VT switch input.Based on the state of provisioning bit 30, the switch determines if theoutput is being sourced from an STS input or a VT input. An example of aVT-mapped input to the switch 52 shows the VT-mapped inputs have theirSPE (i.e. 28 bytes of VT data 56) fixed to an advanced pointer. Forexample, the VT frame inputs have their payload which starts with J1byte rolled back from position 60 a (pointer offset ‘522’) to position54 a (pointer offset ‘492’). After switching, the J1 byte at location 54a is in location 54 b in output 62. Likewise, VT data 56 b in output 62is delayed compared to VT data 56 a in input stream 52. If the input isan STS entry, the data is read out of the data memory one timeslot afterit is written. For example, the overhead 66 a (labeled A1, A2, and J0)and the SPE area 68 a in STS mapped input 64 are each shifted by asingle timeslot (i.e. one STS-1 byte number) to locations 66 b and 68 bin the STS mapped output 70. The adjustment of the pointer for the VTsources results in no apparent frame skew between the VT mapped output62 and the STS mapped output 70 (as shown by line 72).

While in this example the pointer was offset by 30 timeslots, theadvancement or delay of the payload depends on the relative amount ofdelay that a data frame encounters in traveling through the data memoryblock 23. These delays are usually known parameters of the switchingapparatus 20.

While FIGS. 1-4 are described primarily in terms of a SONET system,other system types can be used. For example, SDH networks and/or TUswitching and alignment can be used.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A method for managing latency comprising: receiving data from ahigh-order synchronous transport module (STM) and synchronous transportsignal (STS) sources and low-order tributary unit (TU) and virtualtributary (VT) sources; providing a provisioning bit for each outputassociated with a memory; and adjusting a pointer for the low-ordersources based on the provisioning bit such that the high-order andlow-order outputs are synchronized.
 2. The method of claim 1 furthercomprising determining an adjustment for the pointer based on apredetermined delay.
 3. The method of claim 1 wherein adjusting thepointer includes adjusting the pointer by a predetermined number of timeslots.
 4. The method of claim 1 further comprising assemblingsynchronized outputs from the memory.
 5. The method of claim 1 furthercomprising adjusting the pointer to synchronize the output of thehigh-order and low-order sources, wherein the high-order sources incurless delay to pass through the memory than low-order sources.
 6. Themethod of claim 1 wherein providing a provisioning bit includes storingthe provisioning bit in a connection memory.
 7. The method of claim 1wherein adjusting a pointer for the low-order sources based on theprovisioning bit includes adjusting the pointer when the provisioningbit is set.
 8. The method of claim 7, further comprising setting theprovisioning bit for low-order sources.
 9. The method of claim 1 whereinadjusting a pointer for the low-order sources based on the provisioningbit includes not adjusting the pointer when the provisioning bit is notset.
 10. The method of claim 9 further comprising setting theprovisioning bit for high-order sources.
 11. A computer program producttangible embodied on a computer readable medium, for provisioningcross-connects in network switching environment comprising instructionsfor causing a computer to: receive data from high-order synchronoustransport module (STM) and synchronous transport signal (STS) sourcesand low-order tributary unit (TU) and virtual tributary (VT) sources;provide a provisioning bit for each output associated with a memory; andadjust a pointer for the low-order sources based on the provisioning bitsuch that the high-order and low-order outputs are synchronized.
 12. Thecomputer program product 11 further comprising instructions for causinga computer to: determine an adjustment for the pointer based on apredetermined delay.
 13. The computer program product of claim 11further comprising instructions for causing a computer to: adjust thepointer by a predetermined number of time slots.
 14. The computerprogram product of claim 11 further comprising instructions for causinga computer to: assemble synchronized outputs from the memory.
 15. Thecomputer program product of claim 11 further comprising instructions forcausing a computer to: adjust the pointer to synchronize the output ofthe low-order and high-order sources, wherein the high-order sourcesincur less delay to pass through the memory than low-order sources. 16.The computer program product of claim 11 further comprising instructionsfor causing a computer to: store a provisioning bit in a connectionmemory.
 17. The computer program product of claim 11 further comprisinginstructions for causing a computer to: adjust a pointer for thelow-order sources based on the provisioning bit includes adjusting thepointer when the provisioning bit is set.
 18. The computer programproduct of claim 11 further comprising instructions for causing acomputer to: adjust a pointer for the low-order sources based on theprovisioning bit includes not adjusting the pointer when theprovisioning bit is not set.
 19. An apparatus including: a memory forstoring a provisioning bit for each output associated with a memory; afirst circuit configured to receive data from high-order synchronoustransport module (STM) and synchronous transport signal (STS) sourcesand low-order tributary unit (TU) and virtual tributary (VT) sources; asecond circuit configured to adjust a pointer for the low-order sourcesbased on the provisioning bit such that the high-order and low-orderoutputs are synchronized.
 20. The apparatus of claim 19 wherein thesecond circuit is configured to adjust the pointer by a predeterminednumber of time slots.
 21. The apparatus of claim 19 wherein the secondcircuit is configured to adjust the pointer to synchronize the output ofthe high-order and low-order sources, wherein the high-order sourcesincur less delay to pass through the memory than low-order sources. 22.The apparatus of claim 19 wherein the memory is a connection memory.